Possibly dangerous substances, for example analyte substances, in ambient air are monitored in a large number of applications, such as environmental air analysis in nature or in buildings (e.g., inside chemical companies; for the detection of leaking poisons, chemical warfare agents (CWAs) or explosives). Ion mobility spectrometry is a method introduced in the 1970s for highly sensitive detection of polluting substances at low concentrations in air or other sample gases.
An ion mobility spectrometer (IMS) can be operated at ambient pressure. Ion mobility spectrometers are relatively compact and can be manufactured simply and inexpensively, which makes them particularly suitable for portable and mobile gas monitors and warning devices. Drift-time spectrometers are the most widely used ion mobility spectrometers. Other types of ion mobility spectrometers include, for example, the Aspiration Ion Mobility Spectrometer from the Finnish company Environics Oy and the Asymmetric Field Ion Mobility Spectrometer (FAIMS).
The analyte substances to be detected are usually ionized in a “reaction chamber”. The drift-time IMS includes a measuring tube comprising the reaction chamber and a drift chamber. Ions generated in the reaction chamber, for example short ion pulses, are introduced into the drift chamber. The ion pulses move through the drift gas under the influence of an electric field. The ion pulses are separated in drift times according to their different drift velocities caused by their different mobilities. If the parameters of the drift gas, such as its temperature, humidity and pressure, are held constant, then the drift velocities of the ions are characteristic of the various substances.
In FAIMS devices, the ions are separated according to a dependency of their mobility on the field strength. The following description, however, will concentrate on drift-time ion mobility spectrometers. However, this restriction should not exclude the invention from being used by other types of ion mobility spectrometers.
In the majority of mobile ion mobility spectrometers, the gas is driven around in a closed internal gas circuit by a gas transporting device, such as a gas pump or a fan. A filter incorporated into the gas circuit removes moisture from the circulating gas and purifies it of analyte substances before it is returned again to the measuring tube of the ion mobility spectrometer. In particular, the temperature and humidity of the circulating gas are held constant within the closed gas circuit because these parameters strongly influence the sensitivity of the spectrometer.
The analyte substances which are to be detected in the sample gas may permeate into the gas circuit through a permeable membrane, on which the sample gas impinges from the outside. A membrane inlet usually consists of two chambers separated by the membrane, where the inner chamber is part of the closed internal gas circuit, and the outer chamber is part of an external gas circuit. The outer chamber is flooded through by the sample gas drawn in from a sampling location. Membranes made of organic polymers for example silicone rubber, may be used. The membranes are more permeable to the majority of organic analyte substances than water, thus reducing the unfavorable ingress of moisture into the gas circuit. A membrane inlet requires heating in order to reduce permeation delay and unwanted storage effects in the edges of the membrane where the material is clamped into a frame. In practice, the reaction of the measurement signal to a change in external concentration, in a number of low-volatility substances, may be delayed by several minutes at normal industry temperatures and membrane thicknesses.
Many commercially available ion mobility spectrometers that operate as stand-alone gas detectors incorporate a surge protection system. The circulation of the sample gas in the external gas circuit is interrupted if the measurement signal for a particular analyte substance exceeds a specified limit and the outer side of the membrane is purged with filtered air. Due to the time delays in the membrane inlet, when substance concentrations are high, the membrane and the surfaces in the inlet area of the internal gas circuit are already heavily exposed to the analyte substance before the high concentration is detected in the measuring tube and the surge protection is triggered. As a result, quantities of analyte substance that have already passed through the membrane into the gas circuit, or are still stored in the membrane, still reach the measuring tube even after the surge protection has been triggered. Unheated surfaces in the inlet region of the measuring tube are therefore significantly exposed to the analyte substances and the measurement signals remain saturated for a period of time, known as a dead time. During the dead time, the ion mobility spectrometer is no longer able to perform its warning function nor able to detect other analyte substances. The dead times caused by excessive analyte substance concentrations in the measuring tube may often be significantly longer than the time required to purge the heated membrane. If the concentration of analyte substance is still high enough, the cycle of purging and measurement is repeated, in some examples several times, until the substance concentration has dropped sufficiently.
Ion mobility spectrometers of the prior art have a relatively low dynamic measuring range that is determined by the design and operating parameters of the measuring tube. It is not realistically possible for these parameters to be modified during operation to match changed measuring conditions. It is, however, known that the measuring range of an ion mobility spectrometer can be extended to encompass high substance concentrations where the gas sample drawn in is diluted with a measured quantity of purified air or other gas before it reaches the permeable membrane. When the dilution ratio is changed, it is necessary to allow a settling time to elapse after the change before the measurement signal stabilizes due to the time delays from the membrane inlet. During the settling time, the ion mobility spectrometer does not yield any reliable results.
Commercially available ion mobility spectrometers are also known that monitor the air from several sampling locations synchronously using a single measuring tube. It is often desirable to monitor the gas from several sampling locations in order to reduce the expense and number of devices employed. For example, several inlet lines, usually heated, are used to feed sample gas from the various sampling locations. The gas flowing from the sampling locations are cyclically connected to a single membrane inlet via switching valves. To obtain reproducible measurements, it is necessary, when switching between two sampling locations, to wait until the measurement signal from the previous sampling location has fully decayed and the analyte substances from the following sampling location have permeated the membrane. The sluggishness of the membrane inlet results in relatively long cycle times which seriously restrict the usefulness of such a method.